Phase change materials (PCMs) undergo fast reversible phase changes in response to an external stimulus, such as heat. The phase change is associated with a change in a physical property, such as electrical resistance or optical reflectivity, which can be measured to determine the phase of the material. PCMs are typically switched between a largely amorphous state and a largely crystalline state. The amorphous state is characterized by a higher electrical resistance than the crystalline state. Switching between these two states generates a reversible difference in electrical resistance that can be harnessed for a variety of applications.
Materials that exhibit fast and reversible phase changes are in high demand for many semiconductor applications. Particular applications for this technology include memory cell devices that store binary information. These devices can be classified as either a volatile memory device or a non-volatile memory device. A volatile memory device may lose data stored in the device when power is removed from the device. On the other hand, a nonvolatile memory device may retain its data even without power.
PCM memory devices have the potential to compete with existing memory devices due to their comparatively high resistivity in both the amorphous and crystalline states. High resistivities lead to a high voltage drop and higher power deposition for a given current pulse, which in turn requires less current to switch the cell from the crystalline state to the amorphous state and vice versa.
PCMs commonly applied in this technology include chalcogenide metal alloys composed of germanium (Ge), antimony (Sb) and Tellurium (Te), for example, Ge2Sb2Te5 (GST). Examples of metal alloys utilized as PCMs disclosed in the prior art include: WO 2007/029938 which relates to a phase change memory device using an antimony-selenium metal alloy. The material has a low melting point and high speed of crystallization. US 2007/0001160 discloses a phase change memory material wherein the base material is an antimony-tellurium binary solution, an antimony-germanium binary solution, an antimony-indium binary solution or an antimony-gallium binary solution. The solutions disclosed are in the eutectic range. However, a disadvantage of conventional metal alloy PCMs is their tendency to degrade in high volume switching activity.
One promising approach for the fabrication of resistive nonvolatile memory cells is based on the use of solid solutions as an active (switching) material for nonvolatile memory cells. A memory cell of this type has a layer of a solid solution phase change material arranged between a first electrode and a second electrode.
Solid solution PCMs are known in the prior art. Solis et al (FAST CRYSTALLIZING GeSb ALLOYS FOR OPTICAL-DATA STORAGE Journal of Applied Physics, 1994, vol. 75, N12 June 15, pp. 7788-7794) disclose fast reversible optical storage materials in which amorphous-crystalline cycling is achieved by employing ultra-short laser pulses. A high-reflectivity extended solid solution of germanium in crystalline antimony is also disclosed.
Afonso et al. (ULTRAFAST ERASABLE OPTICAL STORAGE IN Sb-RICH GeSb FILMS The 1994 Conference on Lasers and Electro-Optics Europe, Amsterdam, Netherlands, 28 Aug,-2 Sept., 1994. Publisher: IEEE Piscataway, N.J.) disclose the use of films of solid solutions of germanium and antimony for recording micron-sized bits for optical discs. A specific alloy composition disclosed in the article is 13% germanium in antimony, which is very near the eutectic. Neither of the two articles discloses the use of solid solutions for phase change memory cell devices.
None of the above-cited references, taken either alone or in combination, anticipate the present invention as disclosed and claimed herein.